The proposed study will investigate the effects of chronic tubule obstruction on nephron function and structure. Experiments will be done in anesthetized rats using micropuncture techniques. Specific goals are (1) to examine whether the rapidly-acting tubuloglomerular feedback mechanism is hyper-responsive after one day of ureteral obstruction, (2) to examine the effect of different durations of single nephron blockade on the responsiveness of the rapidly acting tubuloglomerular feedback mechanism, (3) to determine the effect of relieving tubule obstruction on glomerular capillary pressure in low-dose HgCl2-induced acute renal failure, (4) to identify the hormonal mediators responsible for the decreased glomerular blood flow that accompanies chronic tubule blockade, and (5) to study the ultrastructure of chronically blocked versus normal nephrons. These studies should elucidate the mechanisms involved in the impaired glomerular function that occurs with chronic tubule blockade.